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I I I I I I I I I I I I I I I I I I I 5.0 FEASIBILITY DESIGN 5.1 Design Criteria The alternative design concepts and cost estimates were reviewed with the City, and in conjunction with the results of the water analysis discussed in Chapter IV. It was decided to proceed with feasibility-level designs for an enlarged Elkhead Lake Dam, with the capability of providing 2,400 acre-feet of additional reservoir storage and the ability to saf~ly pass the design flood wi thout overtoppi n9 the dam. The rai sed dam is des i gned to all 0;0/ future enlargement should the need arise for additional storage. 5.2 Design Description Genera 1 The primary component of the feasibility design involves excavating an auxiliary spillway channel in the left abutment bedrock and utilizing the excavated materi al s to raise the embankment dam 10 feet. Other major work itens include raising the existing horseshoe-shaped spillway crest five feet to provide additional reservoir storage and extending the concrete spillway walls to contain the higher flow levels. Relocation of the bridge crossing over the existing spillway chute and the left abutment access road will also be requi red. The essential features of the dam modifications are shown in plan on Figure V.4. Embankment Dam Prior to raising the dam, the crest and downstream face will be stripped of gravel surfacing and vegetative materials to expose the internal embankment material. Excavation of a portion of the existing crest will be necessary to expose the six-foot wide chimney filter zone and extend it with the raised dam. The foundation downstream of the toe will be stripped and the clay layer overlying the gravelly sand unit will be excavated below the raised portion of the embankment. The 12-foot wi de fi nger dra i ns beneath the downstream shell will be exposed and extended to the toe of the raised darn. A toe drain tr~nch - 44 -